<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <script src="js/vue-2.4.0.js"></script>
</head>

<body>
    <div id="app">
        <input type="text" v-model="n1">
        <select v-model="n4">
            <option value="+">+</option>
            <option value="-">-</option>
            <option value="*">*</option>
            <option value="/">/</option>
            <input type="text" v-model="n2">
            <input type="button" value="=" @click="hello">
            <input type="text" v-model="n3">
        </select>
    </div>
</body>
<script>
    var vue = new Vue({
        el: "#app",
        data: {
            n1: 1,
            n2: 2,
            n3: 0,
            n4: "-",
        },
        methods: {
            hello() {
                switch (this.n4) {
                    case '+':
                        this.n3 = parseInt(this.n1) + parseInt(this.n2)
                        break;
                    case "-":
                        this.n3 = parseInt(this.n1) - parseInt(this.n2)
                        break;
                    case "*":
                        this.n3 = parseInt(this.n1) * parseInt(this.n2)
                        break;
                    case "/":
                        this.n3 = parseInt(this.n1) / parseInt(this.n2)
                        break;
                };

            }
        }
    })
</script>

</html>